Titanate-based ceramic films have been investigated for their applications in a variety of fields. Making use of their dielectric characteristics, in particular, they have already found utility as electronic members in electronic parts or components such as high-dielectric capacitors, ferroelectric capacitors, gate insulating films, barrier films, capacitors and piezoelectric elements.
Production processes of these films include MOD process, sol-gel process, the process that applies and then fires a dispersion of ceramic particles, CVD process, and ALD process. For thin films that require relatively low processing accuracy, preferred are wet coating processes such as sol-gel process, MOD process and the process that applies and then fires a dispersion of ceramic particles, all of which require low production cost and permit easy film formation. As film-forming raw materials for use in these processes, metal alkoxide compounds, metal salts of organic acids, and fine ceramic particles are employed. These wet coating processes, however, have a limit on the thickness of a film available from a single coating step. An increase in the metal content or viscosity of a coating formulation to make greater the thickness of a film available from a single coating step results in a film of deteriorated quality such as a non-uniform or cracked film. To obtain good film quality, a thickness of from 100 to 300 nm or so is a limit per coating. To overcome this limit, processes that mix fine ceramic particles and a sol-gel solution with each other are disclosed in JP-A-11-502262 (specifically, claims 1, 4 and 5) and JP-A-2005-213105 (specifically, claims 1 and 5 and paragraphs [0018] and [0021]), and a process that mixes fine ceramic particles and an MOD solution with each other is disclosed in JP-A-2005-247632 (specifically, claim 1 and paragraph [0080]).
According to the process disclosed in JP-A-11-502262, a dispersion prepared by mixing an organometal sol-gel solution with fine ceramic powder is employed, and coating and firing are repeated at least 5 times to form a polycrystalline film of at least 10 μm. This patent document also discloses titanium isopropoxide as an organometal titanium compound, and also discloses titania films and PZT films as titanate-based ceramic films.
According to the process disclosed in JP-A-2005-213105, a thin, polycrystalline metal oxide film is produced with a dispersion of fine particles of a corresponding metal oxide in a sol-gel solution prepared by mixing an organometal compound, a metal alkoxide or a salt of an organic acid with an organic solvent. As titanate-based thin films, thin films of strontium barium titanate and thin films of magnesium titanate are disclosed. This patent document, however, contains no disclosure about any titanium compound as a raw material for a sol-gel solution.
JP-A-2005-247632 discloses a dispersion of ultrafine particles of a metal oxide, which has been prepared by mixing, with a dispersion of ultrafine particles of a corresponding titanium-containing complex metal oxide, an MOD solution for the same kind of titanium-containing complex metal oxide. The dispersion of the ultrafine particles of the titanium-containing complex metal oxide was obtained by a hydrolysis in a microemulsion. As an MOD solution, a solution comprising barium n-butyrate, titanium isopropoxide, isoamyl butyrate and 2,4-pentadione is disclosed. No sufficient film-thickening effects can, however, be brought about even by the above-described processes, leading to an outstanding demand for further improvements.
WO-A-2004/097854 (specifically, claim 1, page 5, lines 30-37) discloses a liquid coating formulation, in which ferroelectric oxide particles of plate or needle crystals having a perovskite structure represented by ABO3 (A: Ba, Sr, Ca, Pb, Li, K or Na; B: Ti, Zr, Nb, Ta or Fe) and having an average primary particle size of not greater than 100 nm are dispersed and a soluble metal compound capable of forming a ferroelectric oxide when heated is also dissolved. Disclosed as examples of the soluble metal compound include inorganic salts such as nitrates, organic acid salts such as ethylhexanoate salts, organic metal complexes such as acetylacetone complexes, and metal alkoxides. However, the technology disclosed in this patent document is not to decrease the number of repetitions of coating and firing upon production of a thick film, and this document does not contain any specific description about an organic acid-titanium compound.
Further, JP-A-5-230079 (specifically, claim 1, [0007]) discloses organic acid-titanium compounds represented by the following formula:
wherein each R represents a C1-17 monovalent organic group. These compounds are described to be useful as stable materials in a coating-pyrolysis process for titanium oxide, P(L)ZT and like ferroelectric materials. JP-A-5-58636 (specifically, claim 1, [0012]) discloses a process for the production of lead zirconate titanate, which comprises drying and firing a mixed solution with a titanium salt of an organic acid contained therein. The titanium salts of organic acids, which are described in this patent document, impart stability to coating formulations, but do not act to thicken films.